Buffer layer for GaN-on-Si LED

ABSTRACT

A buffer layer of zinc telluride (ZnTe) or titanium dioxide (TiO 2 ) is formed directly on a silicon substrate. Optionally, a layer of AlN is then formed as a second layer of the buffer layer. A template layer of GaN is then formed over the buffer layer. An epitaxial LED structure for a GaN-based blue LED is formed over the template layer, thereby forming a first multilayer structure. A conductive carrier is then bonded to the first multilayer structure. The silicon substrate and the buffer layer are then removed, thereby forming a second multilayer structure. Electrodes are formed on the second multilayer structure, and the structure is singulated to form blue LED devices.

TECHNICAL FIELD

The present disclosure relates generally to GaN-based blue LEDsmanufactured on silicon substrates and related methods and structures.

BACKGROUND INFORMATION

A Light Emitting Diode (LED) is a solid state device that convertselectrical energy to light. Light is emitted from an active layer ofsemiconductor materials sandwiched between oppositely doped layers whena voltage is applied across the doped layers. There are many differentLED device structures that are made of different materials and havedifferent structures and that perform in different ways. Some emit laserlight, and others generate non-monochromatic and non-coherent light.Some are optimized for performance in particular applications. Some arehigh power devices and others are not. Some emit light as infraredradiation, whereas others emit visible light of various colors, andstill others emit ultraviolet light. Some are expensive to manufacture,whereas others are less expensive. For commercial general lightingapplications, a blue LED structure is often used. Such a blue LED havinga Multiple Quantum Well (MQW) active layer involvingindium-gallium-nitride may, for example, emit non-monochromatic andnon-coherent light having a wavelength in a range from 440 nanometers to490 nanometers. A phosphor coating is then typically provided thatabsorbs some of the emitted blue light. The phosphor in turn fluorescesto emit light of other wavelengths so that the light the overall LEDdevice emits has a wider range of wavelengths. The overall LED devicethat emits the wider range of wavelengths is often referred to as a“white” LED.

Although gallium-nitride substrate wafers are available, they are veryexpensive. The epitaxial layers of commercial blue LEDs are thereforetypically grown on wafers of other types of substrates such as, forexample, sapphire wafers. These other substrates are, however, stillundesirably expensive. Common integrated circuits of the type employedin personal computers are generally fabricated on silicon substrates. Asa result of the high volumes of silicon substrates produced for thecomputer industry, silicon substrates are relatively inexpensive ascompared to sapphire substrates. Moreover, second hand semiconductorprocessing equipment for processing silicon substrate wafers is oftenavailable at low prices due to the fact that integrated circuitfabrication companies frequently upgrade their fabrication facilities inorder to keep up with advances in integrated circuit manufacturingtechnology. It therefore would be desirable from a cost point of view tobe able to fabricate GaN-based LEDs on relatively inexpensive siliconsubstrate wafers and to use the available second-hand semiconductorprocessing equipment for processing such silicon wafers, but there aremany problems with growing high quality GaN epitaxial layers on siliconsubstrates.

Many of the problems associated with growing high quality GaN epitaxiallayers on silicon substrates derive from the fact that the latticeconstant of silicon is substantially different from the lattice constantof GaN. When GaN is grown epitaxially on a silicon substrate, theepitaxial material being grown may exhibit an undesirably high densityof lattice defects. If the GaN layer is grown to be thick enough, thenstress within the GaN layer may result in a type of cracking in thelatter-grown portions of the GaN material. Moreover, silicon and GaNhave different coefficients of thermal expansion. If the temperature ofa structure involving GaN disposed on a silicon substrate is increased,for example, then the silicon material portion of the structure willexpand at a different rate from the rate at which the GaN materialexpands. These different rates of thermal expansive give rise to stressbetween the various layers of the LED device. This stress may causecracking and other problems. Furthermore, it is difficult to grow GaN ona silicon substrate because GaN is a compound material and Si is anelemental material. The transition from nonpolar to polar structure,combined with the substantial lattice mismatch, generates defects. Forthese and other reasons, the epitaxial LED structure portions of mostcommercially-available white LED devices are not grown on siliconsubstrates. Improved processes and structures for fabricating blue LEDson silicon substrates are sought.

The manufacture of blue LEDs grown on silicon substrates also typicallyinvolves wafer bonding. In one prior art process, an epitaxial blue LEDstructure is grown on a non-GaN substrate to form a device waferstructure. A layer of silver is formed on the epitaxial LED structure tofunction as a mirror. A barrier metal layer involving multiple periodsof platinum and titanium-tungsten is then disposed on the silver mirror.The platinum layer in each period is a thin 60 nm layer. Thetitanium/tungsten layer in each period is about 10 nm thick and involvesabout approximately ninety percent tungsten. Five or more such periodsare provided. Once the device wafer structure has been formed in thisway, a carrier wafer structure is wafer bonded to the device waferstructure. The original non-GaN substrate of the device wafer structureis then removed and the resulting wafer bonded structure is singulatedto form LED devices. In this prior art process, a layer of bonding metalis used to wafer bond the carrier wafer structure to the device waferstructure. This bonding metal layer involves a gold/tin sublayer. Whenthe gold/tin sublayer is melted during wafer bonding, tin from thisgold/tin sublayer does not penetrate into the silver layer due to thethickness of the multi-period barrier metal layer and due to a shorthigh temperature cycle being used to melt the bonding metal. This priorart process is recognized to work well.

SUMMARY

In a first novel aspect, a white LED assembly involves a blue LEDdevice. The blue LED device is manufactured by epitaxially growing a LowResistance Layer (LRL) over a silicon substrate. In one example, abuffer layer is grown directly on the silicon wafer substrate, and thena template layer of undoped gallium-nitride is grown directly on thebuffer layer, and then the LRL is grown directly on the template layer.

In one example, the LRL is a superlattice structure involving multipleperiods, where each period is thin (less than 300 nm thick) and involvesa relatively thick gallium-nitride sublayer (for example, 100 nm thick)and a relatively thin undoped aluminum-gallium-nitride sublayer (forexample, 25 nm thick). The bottom sublayer of the LRL is a sublayer ofGaN. The top sublayer of the LRL is also a sublayer of GaN. There arefour undoped aluminum-gallium-nitride sublayers in the LRL.

The blue LED device involves an indium-containing light-emitting activelayer sandwiched between two oppositely doped layers. This structure ofan active layer sandwiched between two oppositely doped layers isreferred to here as a “PAN structure”. The n-type layer of the PANstructure is grown directly on the upper surface of the LRL so that then-type layer is disposed directly on a GaN sublayer of the LRL. Then-type layer may involve periods of gallium-nitride andaluminum-gallium-nitride as well, but the gallium-nitride sublayers ofthe n-type layer are substantially thicker than are the gallium-nitridesublayers of the LRL. In addition, the aluminum-gallium-nitridesublayers of the n-type layer are substantially thinner than are thealuminum-gallium-nitride sublayers of the LRL. Thealuminum-gallium-nitride sublayers of the n-type layer are silicon dopedto have a silicon concentration of more than 1×10¹⁸ atoms/cm³, whereasthe aluminum-gallium-nitride sublayers of the LRL layer are undoped andhave a silicon concentration of less than 1×10¹⁸ atoms/cm³.

In subsequent processing, the face side of the silicon device waferstructure is wafer bonded to a carrier wafer structure involving aconductive carrier. The conductive carrier may, for example, be amonocrystalline silicon wafer that is doped to be conductive. After thiswafer bonding, the original silicon wafer substrate is removed bychemical mechanical polishing and/or by other suitable methods.

In a first specific example, the original silicon wafer substrate, thebuffer layer and the template layer are removed but at least a portionof the LRL layer is left. After the removal step, the exposed surface ofthe LRL that remains is a sublayer of gallium-nitride. Electrodes areadded, and the wafer bonded structure is singulated into individual blueLED devices. Within each blue LED device, the n-type layer of the PANstructure is in direct contact with at least a portion of the LRL layer.The LRL layer has a sheet resistance at the LRL/n-type layer interfacethat is lower than the sheet resistance of the n-type layer at theLRL/n-type layer interface. The sheet resistance of the n-type layer ismore than fifteen ohms per square.

In such a blue LED device, the LRL has two functions. A first functionis that in the process of growing the n-type gallium-nitride layer, thepresence of the LRL reduces the concentration of lattice defects in then-type layer that otherwise would be present. The LRL functions to blockdislocation threads originating in the lower template layer fromextending up and into the n-type layer. A second function is a currentspreading function. The LRL is formed such that a so-calledtwo-dimensional gas of high mobility electrons is present in the layersof the superlattice of the LRL. As a result of this two-dimensionalelectron gas, the sheet resistance of the LRL at the LRL/n-type layerinterface is substantially lower than the sheet resistance of the n-typelayer. The relatively low resistivity LRL facilitates lateral currentspreading on one side of the n-type layer. Current flow through theLRL/n-type layer interface plane during LED operation is therefore moreuniform than it otherwise would be were the LRL not present.

In a second specific example, the original silicon wafer substrate, thebuffer layer, the template layer, and the LRL are all removed. The LRLis entirely removed. Electrodes are added, and the wafer bondedstructure is singulated into individual LED devices. In each LED device,the n-type layer of the PAN structure is not in contact with any portionof the LRL because the LRL was completely removed. In this secondspecific example, the LRL serves the first function of reducing theconcentration of lattice defects in the n-type layer.

In a second novel aspect, a wafer bonding process involves wafer bondinga carrier wafer structure to a device wafer structure by melting a layerof eutectic metal and thereby forming a wafer bonded structure. Prior tothe melting, the device wafer structure involves an epitaxial LEDstructure disposed on a substrate, such as a silicon substrate uponwhich the epitaxial LED structure was grown. The device wafer structurefurther includes a layer of a non-reactive barrier metal disposed overthe epitaxial LED structure. In one example, the layer of non-reactivebarrier metal is a single layer of titanium that is more than 50 nmthick. In one example, the eutectic metal layer involves a first goldsublayer, a gold/tin sublayer, and a second gold sublayer, where thegold/tin sublayer is disposed between the two gold sublayers. When theeutectic metal layer melts, the non-reactive barrier metal layerprevents tin from the eutectic layer from diffusing through thenon-reactive barrier layer. In one specific example, there is a highlyreflective layer of silver disposed between the epitaxial LED structureand the non-reactive barrier metal layer. This silver layer provides amirror function, and also serves as an electrical contact to theepitaxial LED structure. The non-reactive barrier metal layer preventstin from the eutectic bonding metal layer from passing into this silverlayer during the wafer bonding process. Were the tin allowed to diffuseinto the silver mirror, then the reflectivity of the silver mirror mightbe decreased and the contact resistivity of the silver contact might beincreased.

In a first advantageous aspect, a high temperature cycle that melts theeutectic metal layer involves heating the carrier wafer structure to atemperature of more than 280° C. (e.g., 310° C.), and maintaining thistemperature for more than one minute. In a second advantageous aspect,there is at least one layer of platinum disposed between the epitaxialLED structure and the carrier of the carrier wafer structure, and thesum of all the thicknesses of all the layers of platinum between theepitaxial LED structure and the carrier is less than 200 nm. In a thirdadvantageous aspect, there is one and only one layer of platinum betweenthe epitaxial LED structure and the carrier. This platinum layer servesa silver-encapsulating function to prevent the electromigration ofsilver. The platinum encapsulation layer has a thickness of less than200 nm. Wafer bonding results in a wafer bonded structure. After waferbonding, the silicon substrate of the device wafer structure is removed,electrodes are added to the remaining wafer bonded structure, and thewafer bonded structure is singulated to form blue LED devices.

Platinum is a quite expensive metal, whereas titanium is substantiallyless expensive. The prior art wafer bonding process described above inthe background section involves five or more platinum layers, each ofwhich may be 100 nm or more in thickness. The prior art process uses 500nm or more of platinum. By reducing the amount of platinum used to asingle layer that is thinner than 200 nm, the novel non-reactive metalbonding process disclosed here can reduce the cost of manufacturing blueLEDs on silicon substrates.

In a third novel aspect, a blue LED device is manufactured byepitaxially growing an n-type Gallium-Nitride (GaN) layer over a siliconsubstrate using Zinc-Sulfide (ZnS) as a transitional buffer layer. Inone example, the ZnS buffer layer is 50 nm thick, and the n-type GaNlayer is at least 2000 nm thick. Growing the n-type GaN layer on the ZnSbuffer layer reduces lattice defect density in the n-type GaN layer.First, the ZnS buffer layer provides a good lattice constant match withthe silicon substrate and a compound polar template for subsequent GaNgrowth. Second, ZnS can be easily prepared by metal-organic chemicalvapor deposition (MOCVD) such that all the epilayers are grown in onegrowth chamber. Third, the melting point of ZnS is 1850° C., which ishigh enough to keep it from becoming unstable during GaN deposition.Finally, if an aluminum-nitride (AlN) layer is also used as part of thebuffer layer, then the ZnS layer is used as a diffusion barrier betweenthe AlN layer and the silicon substrate. After the epitaxial layers ofthe epitaxial LED structure are formed, a carrier wafer structure iswafer bonded to the structure where the carrier wafer structure includesa conductive carrier. The original silicon substrate and the ZnS bufferlayer are then removed from the wafer bonded structure. Electrodes areadded and the wafer bonded structure is singulated to form finished LEDdevices.

In a fourth novel aspect, a vertical GaN-based blue LED device has ann-type layer that comprises multiple conductive intervening layers. Inone example, the n-type layer contains a plurality of periods, and eachperiod of the n-type layer includes a gallium-nitride (GaN) sublayer andan aluminum-gallium-nitride doped with a silicon (AlGaN:Si) interveningsublayer. In one example, each GaN sublayer has a thickness of 900 nm,each AlGaN:Si intervening sublayer has a thickness less than 25 nm.Because AlGaN has a smaller lattice constant than GaN, the AlGaN:Siintervening layers provide a compressive strain to the GaN sublayers andprevent cracking. After each intervening layer, the quality of theoverlying GaN sublayer is of improved quality in terms of lower densityof lattice defects. In addition, the AlGaN:Si layers are electronicallyconductive (e.g., 1×10⁷ to 1×10⁹ defects per cm³) and have a siliconconcentration greater than 1×10¹⁸ atoms/cm³. After the epitaxial layersof the epitaxial LED structure are formed, a carrier wafer structure iswafer bonded to the device wafer structure where the carrier waferincludes a conductive carrier. The original silicon substrate of thewafer bonded structure is then removed. Electrodes are added, and thewafer bonded structure is singulated to form finished LED devices.Because the AlGaN:Si intervening sublayers are conductive (e.g.,resistivity=1×10⁻²·Ω·cm), they do not need to be removed in the finalLED device. Rather, the entire n-type layer remains in the completedblue LED device and has a thickness of at least two thousand nanometersto provide enhanced current spreading and to provide more n-GaN materialto accommodate surface roughening.

In a fifth novel aspect, a layer of either zinc telluride (ZnTe) ortitanium dioxide (TiO₂) is formed directly on a silicon substrate as abuffer layer. A layer of AlN, or a layer of another suitable bufferlayer material, is then optionally formed over the ZnTe layer as asecond layer of the buffer layer. A template layer of GaN is then formedover the buffer layer. An epitaxial LED structure is formed over thetemplate layer. The epitaxial LED structure is the epitaxial LEDstructure for a GaN blue LED. It therefore has an active layercomprising gallium and nitrogen. The resulting structure is referred toas a first multilayer structure. A conductive carrier is then bonded tothe first multilayer structure. The silicon substrate and the bufferlayer are then removed, thereby forming a second multilayer structure.Electrodes are formed on the second multilayer structure, and thestructure is singulated to form blue LED devices.

Further details and embodiments and techniques are described in thedetailed description below. This summary does not purport to define theinvention. The invention is defined by the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, where like numerals indicate like components,illustrate embodiments of the invention.

FIG. 1 is a cross-sectional diagram of a white LED assembly inaccordance with one novel aspect.

FIG. 2 is a top-down diagram of the white LED assembly of FIG. 1.

FIG. 3 is simplified and expanded cross-sectional diagram of the whiteLED assembly of FIG. 1.

FIG. 4 is a top-down diagram of the blue LED device of the white LEDassembly of FIG. 1.

FIG. 5 is a cross-sectional diagram of a portion of the blue LED deviceof the white LED assembly of FIG. 1.

FIG. 6 is a cross-sectional diagram of the epitaxial PAN structurelayers of the blue LED device of FIG. 3 formed on a silicon substrate.

FIG. 7 is a diagram that illustrates a step in a method of manufacturingwhere current blocking structures are added to the structure of FIG. 6.

FIG. 8 is a diagram that illustrates a step in the method ofmanufacturing where a highly reflective layer is added to the structureof FIG. 7.

FIG. 9 is a diagram that illustrates a step in the method ofmanufacturing where an encapsulation layer is formed over the structureof FIG. 8, and where a non-reactive barrier metal layer is formed overthe encapsulation layer.

FIG. 10 is a diagram that illustrates a step in the method ofmanufacturing where bond metal is added to the structure of FIG. 9.

FIG. 11 is a diagram that illustrates a step in the method ofmanufacturing where a carrier wafer structure is wafer bonded to thestructure of FIG. 10.

FIG. 12 is a graph that shows the temperature cycle of the thermalcompressing wafer bonding process used.

FIG. 13 is a diagram that illustrates a step in the method ofmanufacturing where the wafer bonded structure is flipped.

FIG. 14 is a diagram that illustrates a step in the method ofmanufacturing where the silicon substrate, the buffer layer, and thetemplate layer are removed from the structure of FIG. 12.

FIG. 15 is a top-down diagram of the wafer bonded structure after theremoval step of FIG. 14 and after mesas have been formed.

FIG. 16 is a cross-sectional diagram of FIG. 15 taken along line B-B.

FIG. 17 is a diagram that illustrates a step in the method ofmanufacturing where the surface of the LRL is roughened.

FIG. 18 is a diagram that illustrates a step in the method ofmanufacturing where electrodes are added to the structure of FIG. 17.

FIGS. 19A and 19B together are a table that sets forth details abouteach of the steps in the method of manufacturing of FIGS. 6-18.

FIG. 20 is a cross-sectional diagram of a blue LED device that is formedby the method of manufacture set forth above in connection with FIGS.6-18, except that all LRL is removed in the removal step illustrated inFIG. 14.

FIG. 21 is a cross-sectional diagram of a blue LED device that is formedby the method of manufacture set forth above in connection with FIGS.6-18, except that all LRL is removed and some of the n-type layer isremoved in the removal step illustrated in FIG. 14.

FIG. 22 is a flowchart of a method in accordance with a first novelaspect.

FIG. 23 is a flowchart of a method in accordance with a second novelaspect.

FIG. 24 is a flowchart of a method in accordance with a third novelaspect.

FIG. 25 is a flowchart of a method in accordance with a fourth novelaspect.

FIG. 26 is a cross-sectional diagram of the epitaxial PAN structurelayers of the blue LED device of FIG. 3 in an embodiment in which theepitaxial LED structure was grown over a buffer layer including a layerof zinc telluride (ZnTe) or a titanium dioxide (TiO₂).

FIG. 27 is a diagram of a method in which the epitaxial layers of a blueLED are grown on a silicon substrate using a ZnTe or TiO₂ buffer layer.

DETAILED DESCRIPTION

Reference will now be made in detail to some embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. In the description and claims below, when a first layer isreferred to as being disposed “over” a second layer, it is to beunderstood that the first layer can be directly on the second layer, oran intervening layer or layers may be present between the first andsecond layers. The terms such as “over”, “under”, “upper”, “lower”,“top”, “bottom”, “upward”, “downward”, “vertically”, and “laterally” areused herein to describe relative orientations between different parts ofthe blue LED device being described, and it is to be understood that theoverall blue LED device being described can actually be oriented in anyway in three-dimensional space.

FIG. 1 is a cross-sectional side view of a white light emitting diode(LED) assembly 50. FIG. 2 is a top-down diagram of the white LEDassembly 50. The white LED assembly 50 includes four vertical blue LEDdevices 51-54. A vertical LED device is also sometimes referred to as avertically-contacted LED device to distinguish it from a lateral or alaterally-contacted LED device. The four vertical blue LED devices 51-54are mounted to a metal core printed circuit board (PCB) 19. From thetop-down perspective, the four vertical blue LED devices 51-54 aresurrounded by a retaining ring 20. The retaining ring 20 retains anamount of phosphor 21 that covers the blue LED devices 51-54. A portionof a first metal structure 57 is exposed through a first opening in asolder mask layer 58. Anode electrodes of the blue LED devices 51-54 aresurface mounted via amounts of silver epoxy 18 to the first metalstructure 57 in this first opening. The first opening in the solder masklayer 58 also exposes a portion of a second metal structure 59. Upwardlyfacing cathode electrodes of the blue LED devices 51-54 are wirebondedto the exposed portion of the second metal structure 59. A first pad 60is formed by another opening in solder mask layer 58. A second pad 61 isformed by yet another opening in solder mask layer 58. As illustrated inthe cross-sectional view of FIG. 3, the first and second metalstructures 57 and 59 are portions of a metal layer that is disposed on adielectric layer 62. Dielectric layer 62 is a 35 um to 250 um thicklayer of an epoxy material containing inorganic fillers such as Al₂O₃.This dielectric layer 62 insulates the first and second metal structures57 and 59 from an aluminum or copper base portion 63 of the metal corePCB 19.

FIG. 3 is a simplified cross-sectional side view of the white LEDassembly 50. Only one vertical blue LED device 54 is illustrated in thediagram. The vertical blue LED device 54 includes numerous layers, someof which are illustrated in FIG. 3: a first metal electrode 17, aportion of a Low Resistance Layer (LRL) 4, an n-type gallium-nitridelayer 5, an active layer 7, a p-type gallium-nitride layer 8, an amountof metal including a bond metal layer 13, a conductive carrier 15 and asecond metal electrode 16. Reference numeral 64 identifies a wire bondthat couples the first metal electrode 17 of the blue LED device 54 tothe second metal structure 59. Metal 17 and LRL 4 are conductive andfunction to make electrical contact to the n-type layer 5. All thelayers below p-type layer 8 (including layers 13, 15, 16) are conductiveand function to make electrical contact to the p-type layer 8.

FIG. 4 is a top-down diagram of the vertical blue LED device 54. Thefirst metal electrode 17 has a grid shape.

FIG. 5 is a more detailed cross-sectional diagram of the vertical blueLED device 54 taken along sectional line A-A in FIG. 3. Between theconductive carrier 15 and the p-type gallium-nitride 8 are multiplelayers and structures including: a barrier metal layer 14, the bondmetal layer 13, a barrier metal layer 12, a layer of an encapsulantmetal 11, a highly reflective layer 10, and a current blocking layer 9.The current blocking layer 9 is patterned into current blockingstructures. Between the active layer 7 and the n-type gallium-nitridelayer 5 is a strain release layer 6. When current flows from the secondelectrode 16, through conductive carrier 15, through the metal layers14, 13, 12, 11 and 10, through p-type gallium-nitride layer 8, throughactive layer 7, through strain release layer 6, through n-typegallium-nitride layer 5, through the Low Resistance Layer 4, and to thefirst electrode 17, then non-monochromatic and non-coherent light isemitted from the active layer 7. The emitted light has a wavelength in arange of from approximately 440 nm to approximately 490 nm. The term“non-monochromatic” as it is used herein means that the light has aspectral line width that is substantially wider than the spectral linewidth of the light emitted by a typical laser diode. The spectral linewidth of an LED is typically around 20 nm wide, whereas the spectralline width of a laser diode is typically less than 4.0 nm wide.

FIGS. 6-18 are a set of diagrams that set forth steps in a method ofmanufacturing the blue LED device 54. The diagrams are not to scale, butrather are conceptual diagrams.

FIG. 6 is a cross-sectional diagram that shows the result of severalinitial steps of forming epitaxial layers on a silicon substrate 1.Silicon substrate 1 is a monocrystalline silicon substrate wafer of thetype commonly used in the manufacture of high volume CMOS integratedcircuits. A buffer layer 2 is formed on the silicon substrate. In theillustrated example, the forming of this buffer layer 2 involves firstforming a less than 100 nm thick layer 65 of zinc-sulfide (for example,50 nm) on silicon substrate 1. A 200 nm thick layer 66 ofaluminum-nitride (AlN) is then formed on the ZnS layer 65. A 250 nmthick layer 67 of aluminum-gallium-nitride (AlGaN) is then formed on theAlN layer 66. Although this particular buffer layer involving a layer ofzinc-sulfide is shown, other types of buffer layers can be used. Abuffer layer of a single layer of AlN may, for example, be used. Wherethe ZnS layer 65 is provided, the AlN layer 66 and the AlGaN layer 67illustrated in FIG. 6 are optional.

The manufacturing of vertical LED device 54 involves epitaxially latergrowing a gallium-nitride (GaN) layer (e.g., n-type GaN layer 5) oversilicon substrate 1. The in-plane lattice constants of GaN and Si areaGaN(0001)=3.189 {acute over (Å)}, and aSi(111)=3.840 {acute over (Å)},respectively. As a result, there is a substantial 20.4% in-plane latticemismatch between GaN and Si. This lattice mismatch, combined with alarge difference in the thermal expansion coefficient between GaN and Si(e.g., 56%), makes growing high quality, thick, and crack-free GaN onsilicon substrate a challenge. In general, using a transitional bufferlayer satisfying coincident lattice conditions can alleviate the latticemismatch. For example, an AlN layer 66 (aAlN=3.112 {acute over (Å)}) isoften used as a buffer layer to provide a compression to a GaN epilayer.However, inter diffusion of Al and Si at the interface is severe,resulting in high unintentional doping levels. Moreover, AlN crystalquality is low because the lattice mismatch between AlN and Si is evenhigher than the lattice mismatch between GaN and Si.

To overcome this drawback, ZnS layer 65 is used as a new transitionalbuffer layer. First, Wurzite ZnS compound (aZnS=3.811 {acute over (Å)})has a lattice constant of 0.3811, which is between the lattice constantof GaN and Si, and is closer to the lattice constant of Si. ZnS thusprovides a good lattice constant match with Si and is also a compoundmaterial like GaN. Second, ZnS can be easily prepared by metal-organicchemical vapor deposition (MOCVD) such that all the epilayers are grownin one growth chamber. MOCVD is a chemical vapor deposition method ofepitaxial growth of materials, especially compound semiconductors fromthe surface reaction of organic compounds or metalorganics and metalhydrides containing the required chemicals. In one example, ZnS is grownon a silicon substrate in an MOSCVD chamber by introducing hydrogensulfide with dimethyl zinc (DMZn), under 350° C. growth temperature and100 Torr growth pressure. Third, the melting point of ZnS is 1850° C.,which is high enough to keep it from becoming unstable during GaNdeposition. Finally, if an AlN layer is also used as part of the bufferlayer, then ZnS also serves as a diffusion barrier between AlN of thebuffer and the silicon substrate.

A template layer 3 is then formed on the buffer layer 2. In theillustrated example, template layer 3 is a 1000 nm thick layer ofundoped gallium-nitride.

Rather than growing the n-type layer of the LED directly on the templatelayer, the Low Resistance Layer (LRL) 4 is grown directly on thetemplate layer 3. In the illustrated example, LRL 4 is a superlatticestructure that is structured so that a two-dimensional gas of highmobility electrons is formed in its layers. The superlattice structureincludes multiple periods, where each period is less than 300 nm thick.In one specific example, each period includes a 100 nm thick n-typegallium-nitride sublayer and a 25 nm thick undopedaluminum-gallium-nitride sublayer. In the perspective of theillustration of FIG. 6, a 100 nm thick n-GaN sublayer is disposeddirectly on template layer 3. Also, the uppermost sublayer of the LRL 4is a 100 nm thick n-GaN sublayer. There are five GaN sublayers and fourAlGaN sublayers in the configuration illustrated. The n-GaN layers aredoped with silicon to a concentration of 1×10¹⁸ atoms/cm³. Each of thethinner AlGaN sublayers is strained to the lattice of a thicker GaNsublayer or sublayers.

The n-type GaN layer 5 is then grown on LRL 4. It is difficult to grow athick layer of high quality GaN that has a low lattice defect densityand that does not suffer from cracking and other problems due to thebuildup of stresses in the GaN layer. For example, there exists a largedifference in thermal expansion coefficient (e.g., 56%) between GaN andSi. This thermal mismatch usually causes tensile stress in the GaNepilayer during cool down. When the thickness of the GaN epilayer islarger than 1000 nm, cracks usually occur. Although the exact reasonsfor the buildup of stress as a GaN layer grows thicker are not fullyunderstood, it is empirically known that cracking may be prevented bystopping the growth of the GaN layer just before the buildup of stressin the GaN layer would result in cracking. At this point, a thinintervening layer is grown on the top of the GaN layer. This interveninglayer may, for example, be a 5 nm thick layer of AlN. After theformation of the thin intervening sublayer, another sublayer of GaN isgrown on the intervening layer. This second GaN sublayer is grown to beas thick as possible without the GaN sublayer having too much internalstress. This periodicity is repeated multiple times. After eachintervening layer, the quality of the overlying GaN sublayer is ofimproved quality in terms of lower density of lattice defects. Forexample, typical GaN over Si has a defect density as high as 1×10¹⁰defects per cm³. The defect density of improved quality GaN over Si isin the range from 1×10⁷ to 1×10⁹ defects per cm³.

The use of AlN intervening sublayers, however, leads to one problem. AlNis an insulating material due to its wide bandgap (e.g., 6.2 eV) andtherefore creates a barrier for current transportation in the verticaldirection. As a result, all the sublayers from the silicon substrate tothe top AlN intervening sublayer have to be removed if a vertical LED isto be made. The current spreading function of these layers is thereforenot present in the final LED device. The GaN sublayer over the top AlNsublayer is usually less than 2000 nm, which could cause a currentcrowding problem in LED devices. To solve this problem, a thin AlGaN:Silayer is used as a new intervening sublayer instead of using AlN. First,AlGaN:Si is an n-type material the allows electron transportationthrough it because AlGaN has smaller band energy (e.g., bandgap of 3.4eV to 6.2 eV, depending on the Al concentration) than that of AlN, thusthe activation energy of Si in AlGaN is smaller than that of Si in AlN.As a result, the AlGaN:Si intervening sublayers are electronicallyconductive and do not need to be removed in the final LED device. Oneexample of the resistivity of the conductive AlGaN:Si layer is1×10⁻²·Ω·cm. Second, AlGaN has a smaller lattice constant than GaN andtherefore provides compressive strain to the subsequent GaN sublayersand helps prevent cracking.

In the example of FIG. 6, each of the GaN sublayers of n-type GaN layer5 has a thickness of about 900 nm and is doped with silicon at aconcentration of 5×10¹⁸ atoms/cm³. Each of the intervening sublayers ofAlGaN:Si less than 25 nm in thickness (for example, 5 nm) with a silicondopant concentration of 1×10¹⁸ atoms/cm³. These AlGaN:Si interveningsublayers are relatively conductive, and the n-type GaN interveningsublayers are conductive, so the overall n-type GaN layer 5 caneffectively conduct current in the direction from the LRL/n-type layerinterface toward the active layer. The bottom sublayer of n-type GaNlayer 5 that is in direct contact with LRL 4 is a 900 nm thick GaNsublayer as illustrated. The upper sublayer of the n-typegallium-nitride layer 5 is also a 900 nm sublayer of GaN as illustrated.The entire n-type GaN layer 5 with multiple AlGaN:Si conductiveintervening sublayers is approximately 5000 nm thick, where each n-GaNsublayer is substantially thicker than the n-GaN sublayers of LRL 4.

There is an interface 74 between the n-type GaN layer 5 and theunderlying LRL 4. The n-type GaN layer 5 has a sheet resistance at thisinterface that is greater than 15 ohms per square. The LRL 4 also has asheet resistance at this interface, but the sheet resistance of LRL 4 islower than the sheet resistance of the n-type GaN layer 5. In oneexample, the sheet resistance of LRL 4 can be destructively measured bygrinding away the upper layers of a first LED device to expose a surfaceof LRL 4 at interface 74, and then probing the exposed LRL surface andmeasuring its sheet resistance. In a similar fashion, the sheetresistance of n-type layer 5 can be destructively measured by grindingaway the lower layers of a second LED device to expose a surface ofn-type layer 5 at interface 74, and then probing the exposed n-typelayer surface and measuring its sheet resistance.

Next, a strain release layer 6 is formed on the n-type GaN layer 5. Inone example, strain release layer 5 is 120 nm thick and includes thirtyperiods. Each period includes a first sublayer of In_(x)Ga_(1-x)N, where0≦x≦0.12 and a second sublayer of In_(y)Ga_(1-y)N where 0≦y≦0.12, wherex≠y.

Next, the active layer 7 is formed on the strain release layer 6. Theactive layer 7 is fashioned to emit blue light in the overall blue LEDdevice. In one example, active layer 7 is a 130 nm thick MultipleQuantum Well (MQW) structure that has ten periods. Each period includesa 3 nm thick InGaN sublayer (15% In), and a 10 nm thick GaN sublayer.The active layer 7 emits non-monochromatic light having a wavelength inthe range of approximately 440 nm to approximately 490 nm.

Next, the p-type GaN layer 8 is formed on the active layer 7. In oneexample, the p-type GaN layer is 300 nm thick and has a magnesium dopantconcentration of 1×10²⁰ atoms/cm³. In this example, the upper surface ofp-type GaN layer 8 is formed so that the completed LED device will emitnon-monochromatic light and will include no current focusing ridgestructures of the type used in laser diodes. The n-type layer 5, thep-type layer 8, and all layers in between including active layer 7together are an epitaxial LED structure 80.

FIG. 7 is a cross-sectional diagram that shows a subsequent step in themethod of manufacture. A current blocking layer of an insulatingmaterial is formed on the upper surface of the p-type GaN layer 8. Inone example, the current blocking layer is a patterned 200 nm thicklayer of silicon dioxide. This silicon dioxide layer is deposited andpatterned using photolithographic techniques to form structures thatobstruct current flow in some places more than in other places. Forexample, the large structure 66 of the current blocking layer isdisposed directly under the location where the first electrode 17 willbe formed later in the manufacturing method. The large structure 66 isplaced in this location so that current will not flow through the activelayer directly underneath the first electrode 17, and therefore will notgenerate light directly under the first electrode 17. Had light beengenerated in the active region directly underneath the first electrode,then a large amount of that light might be blocked by the metal firstelectrode 17 from escaping the LED device. The blocked light might bereabsorbed in the LED without ever escaping the LED as usable light. Thecurrent used to generate that blocked light would therefore be wasted.By blocking current from flowing through the portion of the active layerdirectly under the first electrode 17, this current is made to flowthrough the active layer in other places where the resulting light has abetter chance of escaping the LED device as useful light. By blocking anappropriate proportion of each sub-area of the upper surface area of thep-type layer 8, and by properly varying this proportion across fromsub-area to sub-area across the upper surface area of the p-type layer8, the amount of current flowing through each individual sub-area iscontrolled to maximize light output from the LED device as compared tocurrent flow through the overall LED device.

FIG. 8 is a cross-sectional diagram that shows a subsequent step in themethod of manufacture. A highly reflective layer 10 is deposited overthe current blocking layer 9. In one example, highly reflective layer 10is a 200 nm thick layer of silver that makes ohmic contact with thep-type GaN layer 8. This silver layer does not cover the entire LEDdevice wafer. The highly reflective layer 10 is shown covering theentire structure of FIG. 8 because the cross-section of FIG. 8 is across-section of only a part of the LED device along line A-A. Highlyreflective layer 10 serves a first function of acting as a mirror toreflect light and serves a second function of making electrical contactto the p-type layer 8.

FIG. 9 is a cross-sectional diagram that shows a subsequent step in themethod of manufacture. An encapsulation layer 11 covers the silver layer10. In the illustrated example, encapsulation layer 11 is a 100 nm thicklayer of platinum. This layer of platinum blocks the electromigration ofsilver. In one advantageous aspect, this layer of platinum is the onlylayer of platinum in the blue LED device. The sum of all the thicknessesof all platinum layers of the blue LED device (there is only one layerof platinum) is less than 200 nm.

After encapsulation layer 11 is formed, a non-reactive barrier metallayer 12 more than 50 nm thick is formed over the encapsulation layer.In the illustrated example, the non-reactive barrier metal layer 12 is a200 nm thick layer of titanium. The titanium is a barrier in that it isa barrier to the diffusion of tin. Tin, from a bond metal layer to belater applied to the top of the structure, is blocked by the barrierlayer from diffusing into the silver of layer 10. Some other metals suchas platinum, titanium/tungsten, gold and nickel that are sometimes usedas barriers for tin, are actually somewhat reactive with tin. In otherbonding processes that use such reactive metals as barriers, thecombination of the thickness of the reactive metal provided and thelimited time of the high temperature bonding cycle is such that theamount of tin incursion is kept within acceptable levels. Thenon-reactive barrier metal of layer 12, in contrast, is not such areactive metal and in one example is a layer of titanium that is morethan 50 nm thick.

FIG. 10 is a cross-sectional diagram that shows a subsequent step in themethod of manufacture. A 500 nm thick sublayer of gold is formed tocover the top of non-reactive barrier metal layer 12. Reference numeral13 in FIG. 10 denotes this gold sublayer that is one sublayer ofthree-sublayer Au/AuSn/Au sandwich structure of metal layers.

FIG. 11 is a cross-sectional diagram that shows a subsequent step in themethod of manufacture. The conductive carrier 5 is covered with anadhesion and barrier metal layer 14. In the illustrated example, theconductive carrier is a monocrystalline silicon wafer and the adhesionand barrier metal layer 14 is a 200 nm thick layer of titanium. Golddoes not generally bond well to silicon, but titanium does, so thetitanium layer 14 is provided to bond to the conductive silicon carrier15 so that a subsequent sublayer of gold (part of the bond metal layer13) can then in turn bond to the titanium. In addition to this adhesionfunction, the titanium also acts as a barrier to prevent tin and goldfrom diffusing to the silicon surface and degrading the adhesion. Inanother example, the adhesion/barrier metal layer 14 also includes alayer of platinum disposed over the titanium.

A 500 nm thick sublayer of gold is then formed on the adhesion andbarrier metal layer 14, and a 3000 nm thick gold/tin sublayer is formedon the gold sublayer. The 3000 nm thick gold/tin sublayer is 80% goldand 20% tin by weight. These gold and gold/tin sublayers are the othertwo sublayers of the three-sublayer sandwich metal structure 13mentioned above in connection with FIG. 9.

The conductive carrier 15, the barrier metal layer 14, and the gold andgold/tin sublayers 13 together are a carrier wafer structure 68. Thesilicon substrate 1 and the layers formed on it together are a devicewafer structure 69. The carrier wafer structure 68 is wafer bonded tothe gold covered upper surface of the device wafer structure 69.

FIG. 12 is a graph showing the temperature cycle of the thermalcompression wafer bonding process used. Carrier wafer structure 68 ispressed against device wafer structure 69 with a pressure ofapproximately fifty pounds per square inch and the pressed togetherwafers are heated to at least 280° C. In the specific example, thewafers are heated to 310° C. The wafers remain at this elevatedtemperature for a period of more than one minute to ensure there is auniform melting temperature all across the wafers. In the specificexample, the elevated temperature is maintained for five minutes. Theeutectic gold/tin sublayer melts, thereby wafer bonding the carrierwafer structure 68 to the device wafer structure 69. This wafer bondingis represented by arrows 70 in FIG. 11. At a twenty percent tinconcentration, the gold/tin sublayer has a melting point ofapproximately 282° C. Once the gold/tin sublayer melts, however, some ofthe tin diffuses from this sublayer into the gold sublayers on eitherside of the gold/tin sublayer. The concentration of tin in the gold/tinsublayer therefor decreases. A gold/tin layer having a lowerconcentration of tin has a higher melting temperature. For every onepercent that the tin concentration decreases in the gold/tin alloy, themelting temperature of the gold/tin alloy increases by approximately 30°C. Accordingly, after the wafer bonding process has been conducted, theconcentration of tin in the gold/tin sublayer decreases to be belowtwenty percent and the overall wafer bonded structure can thereafterhave its temperature raised to 282° C. without the gold/tin sublayermelting.

FIG. 13 shows a subsequent step in the method of manufacture. Theresulting wafer bonded structure 71 is flipped as indicated by arrow 72.

FIG. 14 shows a subsequent step in the method of manufacture. Thesilicon substrate 1, the buffer layer 2 and the template layer 3 areremoved from wafer bonded structure 71 as indicated by arrow 73. In thepresent example, these layers are removed using chemical mechanicalpolishing (CMP) and reactive ion etching (RIE) techniques such that oneof the GaN sublayers of LRL 4 is present and exposed at the top of thestructure. The entire n-type GaN layer 5 is left as part of the waferbonded structure 71. Due to the conductivity of the AlGaN:Si interveningsublayers within layer 5, the n-type GaN layer 5 is electricallyconductive from the strain release layer to n-type layer interface, allthe way through the n-type GaN layer, and to the n-type layer/LRLinterface. An advantage of using AlGaN:Si for the intervening sublayersas opposed to using AlN intervening sublayers is that the AlGaN:Siintervening sublayers are conductive. There is no nonconductiveintervening AlN sublayer within the n-type GaN layer that must beremoved in the step of FIG. 13 in order to provide conductivity throughthe n-type layer. Rather, the entire n-type GaN layer 5 remains in thecompleted blue LED device.

FIG. 15 is a top-down diagram of the wafer bonded structure 71. Inaddition to removing the layers 1, 2 and 3 as illustrated in FIG. 14,wet etching is used to etch horizontal and vertical streets down to theencapsulation layer 11, thereby forming a two-dimensional array of rowsand columns of mesa structures. Reference numerals 75, 76 and 77identify three such streets. Reference numerals 78 and 79 identify twoof the mesa structures.

FIG. 16 is a cross-sectional diagram of the structure of FIG. 15 takenalong sectional line B-B. The non-reactive barrier layer 12 of titaniumis shown in cross-hatching. Non-reactive barrier layer 12 is a singlelayer of titanium more than 50 nm thick and includes no sublayers, noplatinum, and no tungsten. Silver of the highly reflective layer 10 isblocked by the encapsulation layer 11 from moving laterally out fromunder the mesas. The silver of the highly reflective layer 10 iscontained from the top by the bottom surface of the p-type GaN layer 8(the bottom of the mesas).

FIG. 17 shows a subsequent step in the method of manufacture. The uppersurface of the exposed LRL 4 at the top of each mesa is roughened. Thisroughening is the type of roughening customarily performed to facilitatethe escape of light from the LED device. The roughening is actually moresevere than is illustrated in FIG. 17. The vertical distance from thehighest peak to the deepest valley of the roughened surface isapproximately 500 nm, so the deepest valleys extend down into the n-typelayer 5. It is desirable that the LRL not be penetrated, so in anotherexample the amount material removed is preferably limited to removingthe silicon substrate and part of the buffer layer.

FIG. 18 shows a subsequent step in the method of manufacture. The secondelectrode 16 is formed on the conductive carrier 15 as illustrated. Inone example, second electrode 16 includes a first 20 nm thick nickelsublayer that is in direct contact with the conductive carrier 15, andalso includes a second 200 nm thick gold layer that covers the nickelsublayer. Second electrode 16 is therefore 220 nm thick.

In addition, FIG. 18 shows a subsequent step of forming the firstelectrode 17 on the upper surface of LRL 4 as illustrated. In oneexample, first electrode 17 includes a 20 nm thick sublayer of titaniumdisposed directly on LRL 4, a 100 nm thick sublayer of aluminum disposedon the titanium sublayer, a 20 nm thick sublayer of nickel disposed onthe aluminum sublayer, and a 200 nm thick sublayer of gold disposed onthe nickel sublayer. First electrode 17 is therefore approximately 340nm thick. The dopant concentration in the GaN sublayers of LRL 4 isadequately high that good ohmic contact is formed between metal 17 andLRL 4.

After the first and second electrodes have been added as shown in FIG.18, the wafer bonded structure is singulated into separate blue LEDdevices. Singulation is performed by sawing the wafer bonded structuredown the streets illustrated in FIG. 15 so that each mesa structurebecomes a separate blue LED device. In the illustrated example, one ofthe resulting blue LED devices is then incorporated into the white LEDassembly of FIGS. 1-3. The second electrode 16 is glued down to themetal core PCB 19 as illustrated in FIG. 3 using a layer of silver epoxy18. The first metal electrode 17 is wirebonded via bond wire 64 to thesecond metal structure 59 of metal core PCB 19 as illustrated. Afterwirebonding, the retaining ring 20 of silicone is formed on thestructure by silk screening. Alternatively, the retaining ring 20 is cutand shaped to the right dimensions, and then applied. Ring 20 is 0.5 to3.0 mm high, and 0.2 mm to 1.5 mm wide. After the silicone has cured,the amount of phosphor 21 is dropped onto the blue LED device 54 so thatit is retained by ring 20. The phosphor 21 is allowed to cure to formthe completed white LED assembly 50.

FIG. 19 is a table that sets forth details about each step of the methodof manufacture. The numbers in the left column of the table are thereference numerals for the various layers of the white LED assembly 50.

FIG. 20 is a cross-sectional diagram of a blue LED device 100 that isformed by the method of manufacture set forth above in connection withFIGS. 6-18, except that all of LRL 4 is removed in the removal stepillustrated in FIG. 13. All or substantially all of the n-type layer 5remains in the finished blue LED device 100. FIG. 17 is not to scale,but rather is a conceptual diagram.

FIG. 21 is a cross-sectional diagram of a blue LED device 200 that isformed by the method of manufacture set forth above in connection withFIGS. 6-18, except that all of LRL 4 is removed and some of the n-typelayer 5 is removed in the removal step illustrated in FIG. 13. All ofthe thin intervening sublayers of the n-type layer are removed, leavingonly the last 900 nm thick GaN sublayer. This last GaN sublayer remainsin the finished blue LED device 200. FIG. 18 is not to scale, but ratheris a conceptual diagram.

FIG. 22 is a flowchart of a method 300 in accordance with a first novelaspect. A superlattice structure is formed (step 301) over a siliconsubstrate. The superlattice structure includes a plurality of periods.Each period is less than 300 nm thick and includes a GaN sublayer. Inone example, a buffer layer is formed on the silicon substrate, and atemplate layer is formed on the buffer layer, and the superlatticestructure is formed on the template layer. Next, an n-type GaN layer isformed (step 302) directly on the superlattice structure. An activelayer containing indium is formed (step 303) over the n-type layer. Ap-type GaN layer is formed (step 304) over the active layer. The siliconsubstrate, the superlattice structure, the n-type layer, the activelayer, and the p-type layer together are a first structure. A conductivecarrier is bonded (step 305) to the first structure thereby forming asecond structure. The silicon substrate is then removed (step 306) fromthe second structure thereby forming a third structure. Electrodes areformed (step 307) on the third structure. The third structure is thensingulated (step 308) thereby forming a blue LED device.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. LRL 4 can be fabricated in numerous different suitableways. In one example, LRL 4 involves periods, where each period includesa first aluminum-gallium-nitride layer and a secondaluminum-gallium-nitride layer, where the aluminum concentrations in thetwo sublayers differ from one another. The composition of the twosublayers is given by Al_(x)Ga_(1-x)N/Al_(y)Ga_(1-y)N, where x and y aredifferent nonzero numbers. Although a specific example is describedabove in which the aluminum-gallium-nitride and gallium-nitridesublayers of the LRL are of different thicknesses, in other examples thesublayers are of substantially the same thickness. In an AlGaN/GaN orAlGaN/AlGaN superlattice the aluminum concentration in one sublayercontaining aluminum can be graded. The LRL can be an AlInN/GaNsuperlattice. The LRL can be an AlGaN/AlGaN/GaN superlattice where eachperiod involves three sublayers.

Although the wafer bonding process involving a non-reactive barriermetal layer is described above in connection with an LED device thatafter wafer bonding is glued using silver epoxy to a metal core PCB toform the completed white LED assembly, the wafer bonding process isusable where an LED is provided with a gold/tin layer for dieattachment. Due to the decreased concentration of tin in the gold/tinsublayer of metal bonding layer 13 during wafer bonding, the meltingtemperature of the metal bonding layer 13 is higher than 280° C. afterthe wafer bonding process has been performed. Accordingly, a completedLED device can be heated to a high enough temperature to melt an amountof gold/tin provided for die attachment purposes without melting themetal bonding layer 13 within the LED device itself. Although the waferbonding process is described above in connection with an example inwhich the eutectic layer is a gold/tin layer, the wafer bonding processis not limited to require a gold/tin eutectic layer. In other examplesthe eutectic layer is a type of metal layer such as, for example, agold/indium metal layer, and a palladium/indium metal layer. Althoughthe wafer bonding process is described above in connection with anexample where the silver encapsulation layer is platinum, otherencapsulation layers such as nickel and rhodium may be employed.

FIG. 23 is a flowchart of a method 400 in accordance with a second novelaspect. An epitaxial LED structure is grown (step 401) on a non-GaNsubstrate. The non-GaN substrate in one example is a silicon substrateand appropriate intervening buffer and template layers are employed. Asilver layer is then provided (step 402) over the epitaxial LEDstructure to function as a mirror and to function as an ohmic contact tothe epitaxial LED structure. An encapsulating layer is provided (step403) over the silver layer. In one example, the encapsulating layer is asingle layer of platinum that is less than 200 nm thick. A layer ofnon-reactive barrier metal is provided (step 404) over the encapsulatinglayer, thereby forming a device wafer structure. In one example, thisnon-reactive barrier metal layer is a single layer of titanium that ismore than 50 nm thick. This single layer of titanium layer includessubstantially no tungsten.

A carrier wafer structure includes a conductive carrier. The conductivecarrier may be conductive silicon wafer. The carrier wafer structure isthen wafer bonded (step 405) to the device wafer structure by melting aeutectic metal layer between the two wafer structures, thereby forming awafer bonded structure. In one example, the eutectic metal layerinvolves a gold/tin sublayer having a melting temperature ofapproximately 282° C. This eutectic metal layer is melted by raising thetemperature of the carrier wafer structure to more than 280° C. (forexample, to 310° C.) and maintaining this elevated temperature for morethan one minute.

In one example, the temperature of the carrier wafer structure isindirectly determined by placing an amount of gold/tin of the samecomposition of the eutectic bond metal layer in a fully loaded furnacechamber, and slowly increasing the set point of the furnace chamberuntil the gold/tin is observed to melt. This set point is assumed tocorrespond to a carrier device wafer temperature of 282° C. The setpoint of the furnace chamber is then increased an additional amount thatis known to correspond to an increased furnace chamber temperature of30° C. The wafer bonding process is carried out by using this increasedfurnace chamber setting for at least one minute without actuallydirectly measuring the temperature of the wafer bonded structure withinthe furnace chamber.

After wafer bonding, the non-GaN substrate of the resulting wafer bondedstructure is then removed (step 406). Electrodes are formed on the waferbonded structure (step 407), and the wafer bonded structure issingulated thereby forming a plurality of blue LED devices (step 408).In one example of the method 400, the total thickness of any and allplatinum layers in the completed blue LED devices is less than 200 nm,and the high temperature bond metal melting cycle (when the temperatureof the carrier wafer structure is greater than 280° C.) is more than oneminute.

FIG. 24 is a flowchart of a method of manufacturing an LED device onsilicon substrate using zinc-sulfide (ZnS) as a buffer layer inaccordance with a third novel aspect. A zinc-sulfide (ZnS) layer isformed (step 501) over and directly on a silicon substrate. In oneembodiment, the ZnS layer is a buffer layer formed on the siliconsubstrate, and a template layer is then formed on the buffer layer.Optionally, an aluminum-nitride (AlN) layer is formed over thezinc-sulfide layer, and an aluminum-gallium-nitride (AlGaN) layer isformed over the aluminum-nitride layer. The ZnS layer, the AlN layer,and the AlGaN layer form the buffer layer. Next, an n-type GaN layer isformed (step 502) over the template layer. An active layer containingindium is formed (step 503) over the n-type layer. A p-type GaN layer isformed (step 504) over the active layer. The n-type layer, the activelayer, and the p-type layer together are an epitaxial LED structure. Thesilicon substrate, the ZnS layer, and the epitaxial LED structuretogether are a first structure. In one example, the first structure isdevice wafer structure 69 of FIG. 11. A conductive carrier is thenbonded (step 505) to the first structure, thereby forming a secondstructure. In one example, the carrier is part of a carrier waferstructure such as carrier wafer structure 68 of FIG. 11. The originalsilicon substrate and the ZnS layer are then removed (step 506) from thesecond structure, thereby forming a third structure. Electrodes areformed (step 507) on the third structure. The third structure is thensingulated (step 508), thereby forming a blue LED device.

FIG. 25 is a flowchart of a method of manufacturing a blue LED device onsilicon substrate in accordance with a fourth novel aspect. The blue LEDdevice has an n-type gallium-nitride layer with multiple conductiveintervening sublayers. An n-type layer is formed (step 601) over asilicon substrate. The n-type layer comprises a plurality of periods;each period of the n-type layer includes a gallium-nitride (GaN)sublayer and an aluminum-gallium-nitride doped with silicon (AlGaN:Si)intervening sublayer. The AlGaN:Si intervening sublayers are doped withSi and are electronically conductive. Next, an active layer containingindium is formed (step 602) over the n-type layer. A p-type GaN layer isformed (step 603) over the active layer. The silicon substrate, then-type layer, the active layer, and the p-type layer together form afirst structure. A conductive carrier is bonded (step 604) to the firststructure, thereby forming a second structure. The silicon substrate isthen removed (step 605) from the second structure, thereby forming athird structure. Electrodes are formed (step 606) on the thirdstructure. The third structure is then singulated (step 607) to form ablue LED device.

Buffer Layer of ZnTe or TiO₂

FIG. 26 is a cross-sectional diagram of the epitaxial PAN structurelayers of another example of the blue LED device of FIG. 3. Thestructure of the example of FIG. 26 is identical to the structureillustrated in FIG. 6 except that the layer 65 of the example of FIG. 6is a layer 65A of zinc telluride (ZnTe) or titanium dioxide (TiO₂) inthe example of FIG. 26. Layer 65A is less than 100 nm thick.

In the case of layer 65A being a ZnTe layer, the layer 65A is asubstantially homogenous layer of ZnTe disposed directly on the uppersurface 65B of silicon substrate 1. In the case of layer 65A being aTiO₂ layer, the layer 65A is a substantially homogenous layer of TiO₂disposed directly on the upper surface 65B of silicon substrate 1. Theepitaxial LED structure 80 of the example of FIG. 6 is an epitaxial LEDstructure for a blue LED and has an active layer comprising gallium andnitrogen. Epitaxial LED structure 80 is formed over the layer 65A sothat layer 65A is disposed between the epitaxial LED structure 80 andthe silicon substrate 1.

As indicated in FIG. 26, the layers 66 and 67 are optional and may ormay not be provided as part of the overall buffer layer 2 in aparticular embodiment. Accordingly, in one example, the layer 65A is thebuffer layer 2 and the buffer layer 2 includes no additional layers orsublayers. Layer 65A is simply referred to as “the buffer layer”. Incases where AlN layer 66 is provided as part of the buffer layer 2, theZnTe or TiO₂ layer 65A provides a diffusion barrier to help preventsmall amounts of diffusion of aluminum from the AlN layer into thesilicon of the substrate. Small amounts of such diffusion may compromisethe AlN layer in minor ways and may result in the n-GaN of the templatelayer 3 having a lower crystal quality than it would otherwise have.

In one case, silicon substrate 1 is a monocrystalline silicon waferhaving the (111) or (100) crystal orientation relative to its topsurface. In another case, silicon substrate 1 is a monocrystallinesilicon wafer (211) of the type used in the manufacture of MCT (mercurycadmium telluride) infrared photon detectors as described in U.S. PatentApplication Publication US2010/0140735 (the entire subject matter ofwhich is incorporated herein by reference). In examples where layer 65Ais ZnTe, the ZnTe film may be Wurzite ZnTe deposited on the siliconwafer by MOCVD at a substrate temperature of 400° C. to 600° C., usingdimethylzinc (DMZn) and diisopropyltelluride (DIPTe) as sourcematerials. Each aZnTe lattice spacing of 6.10 {acute over (Å)}corresponds to two aSi(111) lattice spacings of 3.84 {acute over (Å)}.The aZnTe lattice spacing of 6.10 {acute over (Å)} divided by two is3.05 {acute over (Å)}, which is smaller than the GaN lattice constant of3.189 {acute over (Å)}. The ZnTe buffer layer therefore providescompressive strain to the GaN on it and relieves tensile strain in theoverlying GaN. The 3.05{acute over (Å)} spacing is smaller than thelattice constant of AlN, so the ZnTe buffer provides even morecompressive strain to the GaN layers than would a conventional AlNbuffer.

In examples where layer 65A is TiO₂, the TiO₂ film may be deposited onthe silicon wafer by MOCVD at a substrate temperature of 400° C. to 600°C. using titanium tetraiso-propoxide Ti(OC₃H₇)₄ (TTIP) as a precursorand nitrogen as a carrier gas. Each TiO2 lattice spacing of 3.05 {acuteover (Å)} corresponds to one aSi(111) lattice spacing of 3.84 {acuteover (Å)}. The TiO₂ lattice spacing of 3.05 {acute over (Å)} is smallerthan the GaN lattice constant of 3.189 {acute over (Å)}. The TiO₂ bufferlayer therefore provides compressive strain to the GaN on it to relievetensile strain in the GaN. The 3.05 {acute over (Å)} spacing is smallerthan the lattice constant of AlN, so the TiO₂ buffer provides even morecompressive strain to the GaN layers than would a conventional AlNbuffer.

The forming of layer 65A is not limited to these deposition methods, butrather other deposition methods known in the art for the forming of zinctelluride and titanium dioxide layers can be employed. Layer 65A neednot be pure ZnTe or pure TiO₂, but rather the chemical composition oflayer 65A may involve other constituents and other compounds of zinc andtellurium or titanium and oxygen. A ZnTe layer may, for example, bedoped with antimony (Sb).

Once the structure of FIG. 26 has been fabricated, subsequent processingas described above in connection with FIGS. 7-18 is carried out torealize the white LED assembly 50 as illustrated in FIGS. 1-3.

FIG. 27 is a flowchart of a method 700 in which epitaxial layers of ablue LED are grown on a silicon substrate using ZnTe or TiO₂ bufferlayer. In a first step (step 701), a buffer layer of zinc telluride(ZnTe) or titanium dioxide (TiO₂) is formed directly on a siliconsubstrate. Optionally, a layer of AlN or a layer of another suitablebuffer layer material is formed on the ZnTe or TiO₂ layer as a secondlayer of the buffer layer. Optionally, a GaN template layer (forexample, u-GaN) is then formed over the buffer layer. Next, an epitaxialLED structure for a blue GaN-based LED is formed (step 702) over thebuffer layer so that the buffer layer is disposed between the siliconsubstrate and the epitaxial LED structure, thereby forming a firstmultilayer structure. A conductive carrier is then bonded (step 703) tothe first multilayer structure. The first multilayer structure comprisesan active layer (part of the epitaxial LED structure), the siliconsubstrate, and the buffer layer. The epitaxial LED structure is for ablue LED and the active layer comprises gallium and nitrogen. In a nextstep (step 704), the silicon substrate is removed from the firstmultilayer structure, thereby forming a second multilayer structure.Electrodes are formed (step 705) on the second multilayer structure. Thesecond multilayer structure is then singulated (step 706) to form a blueLED device.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. An apparatus, comprising: a silicon substrate; abuffer layer disposed directly on the silicon substrate and comprisingan aluminum nitride (AlN) layer and a zinc telluride (ZnTe) layer formedbetween the silicon substrate and the AlN layer; a gallium nitride (GaN)template layer disposed on the buffer layer; and an epitaxial lightemitting diode (LED) structure disposed on the GaN template layer,wherein the buffer layer is disposed between the silicon substrate andthe epitaxial LED structure, wherein the epitaxial LED structurecomprises an active layer disposed between a p-type layer and an n-typelayer, and wherein the active layer comprises gallium and nitrogen. 2.The apparatus of claim 1, wherein the ZnTe layer is a singlesubstantially homogenous layer of zinc telluride (ZnTe).
 3. Theapparatus of claim 1, wherein the buffer layer is less than one hundrednanometers thick.
 4. The apparatus of claim 1, wherein the n-type layerof the epitaxial LED structure has a thickness of at least two thousandnanometers.
 5. The apparatus of claim 1, wherein the ZnTe layer isdirectly disposed on the silicon substrate.
 6. The apparatus of claim 1,wherein the buffer layer further comprises an aluminum gallium nitridelayer disposed between the AlN layer and the GaN template layer.
 7. Theapparatus of claim 1, wherein the GaN template layer is an undoped GaNlayer.
 8. An apparatus, comprising: a silicon substrate; a buffer layerdisposed directly on the silicon substrate and comprising an aluminumnitride (AlN) layer and a titanium dioxide (TiO₂) layer formed betweenthe silicon substrate and the AlN layer; a gallium nitride (GaN)template layer disposed on the buffer layer; and an epitaxial lightemitting diode (LED) structure disposed on the GaN template layer,wherein the buffer layer is disposed between the silicon substrate andthe epitaxial LED structure, wherein the epitaxial LED structurecomprises an active layer disposed between a p-type layer and an n-typelayer, and wherein the active layer comprises gallium and nitrogen. 9.The apparatus of claim 8, wherein the TiO₂ layer is a singlesubstantially homogenous layer of titanium dioxide (TiO₂).
 10. Theapparatus of claim 8, wherein the buffer layer is less than one hundrednanometers thick.
 11. The apparatus of claim 8, wherein the TiO₂ layeris directly disposed on the silicon substrate.
 12. The apparatus ofclaim 8, wherein the buffer layer further comprises an aluminum galliumnitride layer disposed between the AlN layer and the GaN template layer.13. The apparatus of claim 8, wherein the GaN template layer is anundoped GaN layer.